1. Field of the Invention
This invention relates to an improved apparatus for measuring the light emitted from a semiconductor light emitting element such as a light emitting diode.
2. Description of the Related Art
In general, the outputs of semiconductor light emitting elements, such as light emitting diodes, are measured after the elements are manufactured in the form of pellets, thereby to reject the pellets whose outputs are insufficient, or to classify the pellets in accordance with their outputs measured.
FIG. 1 shows a conventional light-measuring apparatus. In the figure, a light emitting element 1, such as a light emitting diode, is provided with an ohmic electrode 2, 4, the ohmic electrode 2 contacts a flat electrode plate 3 incorporated in the apparatus, and another ohmic electrode 4 of the element 1 contacts a probe 5 incorporated in the same. The apparatus has a power source 6 comprising a current source circuit 6A and an ammeter 6B, and interposed between the electrode plate 3 and probe 5, for supplying a predetermined amount of electric current to the light emitting element 1.
A light receiving element 7, such as a photodiode, having a diameter of 10-20 mm is located above the light emitting element 1, for converting the light emitted from the element 1 into an electric signal. The output of the element 7 is measured by a voltmeter 8A, which constitutes a measuring circuit 8 together with a resistor 8B.
FIG. 2 shows another conventional light-measuring apparatus, which is different from the above-described apparatus only in that a transparent electrode layer 9 is used in place of the probe 5. More specifically, the light receiving element 7 has a transparent insulating package 7A. The transparent and conductive electrode layer 9 made of, for example, stannic oxide is formed on the package 7A. The layer 9 is connected to the power source 6. When the layer 9 contacts the ohmic electrode 4 of the light emitting element 1 as is shown in the figure, the element 1 emits light. The light receiving element 7 converts the light into an electric signal, which is measured by the measuring circuit 8.
Referring back to FIG. 1, if the light emitting element 1 has a width of about 0.3 mm, the distance between the elements 1 and 7 must be set to not less than 10 mm, so as to provide a sufficient space for the probe 5. Accordingly, the light receiving element 7, having a small light receiving surface, can receive only a small part of the light emitted from the element 1, which makes it difficult to accurately detect the light output from the element 1.
In the apparatus shown in FIG. 2, the light receiving surface of the element 7 is close to the light emitting element 1, improving the measuring accuracy. However, the electrode layer 9 has sheet resistance .rho.s of about 100 .OMEGA./.quadrature., which is lower than that of any metal. Hence, this apparatus is disadvantageous in that the output of the light emitting element 1 may be varied by the heat the electrode layer 9 generates from the electric power supplied thereto.
The apparatus shown in FIG. 2 is also disadvantageous in another respect. When this apparatus measures the forward-direction voltage characteristic of the light emitting element 1, the measurement may be affected by the voltage drop across the electrode layer 9 due to low conductivity thereof.